Find Where Y=cosx is Decreasing: -π to π

[rocomath]

Hmm ... I did this problem for a friend.

From what x values is y=cos x decreasing from -\pi\leq x\leq\pi

http://img440.imageshack.us/img440/3876/baocu5.jpg​

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*it should be -\frac{\pi}{2} \ \mbox{not} \ -\frac{3\pi}{2}

It's decreasing from (0,\pi) but the answer she gave me from the back of the book is -\frac{3\pi}{2}<x<\frac{3\pi}{2}

 

[rocomath]

Nvm ... we apparently moved onto a different question, lol.

 

[Feldoh]

The book's wrong

 

[montoyas7940]

Must be some of that new math

 

[rocomath]

The book's wrong

LOL, no she started asking about a different question ... I just didn't realize, ha. I was like wtf ... b/c the answer she gave me wasn't even in the interval they were asking about! haha, I kept saying no it's wrong! And I'll prove it by asking the PF Gods :)

 

[Feldoh]

It looks a bit weird when it askes for the answer on the interval [-pi,pi] the books gives the answer as [-1.5pi,1.5pi]